Binding of glycoprotein gpl20 to the T cell-surface receptor CD4 is a crucial step in CD4-dependent infection of a target cell by the human immunodeficiency virus (HIV). Earlier work done in our laboratory has revealed the entry process to a complex, prolonged event requiring some type of cooperative receptor-ligand interactions. It is this phase of viral entry which now can be targeted for vaccine design. Blocking some or all gpl20 molecules on the viral surface should therefore inhibit infection; however, little is known about the molecular mechanisms. We have now quantitatively examined blocking by soluble CD4 in the hope of gaining insight into the complex process of viral blinding, adsorption and penetration. At low sCD4 concentrations, the inhibition in three HIV strains is proportional to the binding of gpl20. For all three viral strains, the biological K(assoc) from infectivity assays is comparable to the chemical K(assoc). The inhibitory action of sCD4 at high concentrations, however, is not fully explained by simple proportionality with the binding to gpl20. Positive synergy in blocking of infection occurs after approximately one-half the viral gpl20 molecules are occupied, and is identical for all three viral strains, despite the large differences in K(assoc). Furthermore, it was shown that a 20- to 50-fold difference in blocking activity could be observed for HIV-1 due to the content of the gpl20/virion and the target-cell density. Thus, unappreciated variations in HIV stocks and assay conditions may hinder comparisons of blockers from laboratory to laboratory, and the age of HIV challenge stocks may influence studies of drug, vaccine efficacy, and antigenic variation studies. The results also suggest that blocking of viral particles in lymphoid compartments will require very high competitive blocker concentrations, which may explain the refractory outcomes from sCD4-based drug trials in humans.